1. Field of the Invention
This invention relates to the fabrication of cellulosic sheets, and more particularly to a process for the fabrication of book cover stock from an admixture of wood-based cellulosic fibers obtained by the chemical pulping of hardwoods and softwoods and the product produced thereby.
2. Description of Prior Art
In the manufacture of bookcovering materials, such as for menu covers, presentation folders, pocket appointment calendar covers and like materials, the base material is a mixture of natural fibers, such as cotton fibers and cellulosic fibers obtained by chemical pulping techniques impregnated with a saturant, such as carboxylated SBR resins added to the fiber mixture prior to forming a sheet substrate. Attempts to prepare sheets solely of like hard and strength qualities from natural fibers containing mixtures of wood-based cellulosic fibers have been unsuccessful due to the required use of long fibers for strength which is inconsistent with requirements of smoothness and uniformity.
Generally, book cover stock is formed on a paper making machine having a cylinder headbox using combined highly refined bleached pulp and cotton fibers (or rag) of up to 80 percent wherein the blended pulp is comprised of not less than about 50 percent long fiber. Refining or fibrillation is required in order to obtain the desired levels of smoothness since softwood fibers are longer and of larger diameter than hardwood fibers. Saturant or resin is added in an amount of from about 20 to 50% by weight with precoating applied to one or both sides of the resulting sheet prior to calendering. The numerous processing steps and concomitant expense of raw materials result in costly product lacking cross-sectional uniformity, exhibiting non-uniform coating capabilities and the like as a result of a high content of long fibers.